Okay. What you are looking at is a Paypal Item Not As Described Dispute.
Send the wrong item back using Expedited Parcel which is Delivery Confirmed and insured up to $100 against loss or damage in the mail.
Open the PP Dispute immediately because you only have 45 days to do so.
Do not wait for a replacement. Ask for your money back.
Once you have the money you can decide whether to start a new transaction with these sellers or find someone more worthy of your trust.
You will not be refunded the cost of your return shipping.
If your seller is overseas, Delivery Confirmed shipping starts around $40 for ExpressPost International and you may be best to just sell on the incorrect item and try to get your money back that way.